Pumping oil wells are normally pumped with a string of sucker rods. The standard equipment installed in the field is ordinarily a walking beam connecting with a polished rod which in turn connects to the string of sucker rods. Sucker rods come in standard lengths. The last two or three sucker rods, however, typically must be cut short to enable the sucker rod string to land the pumping element at a specified depth in the well.
Consider a well of 10,000 feet in depth. Elongation of 0.10% in the sucker rod string corresponds to ten feet of elongation. Consider a well which has a depth wherein the sucker rod string at the time of installation requires the last sucker rod to be shortened by twelve feet. The two factors just mentioned (sucker rod elongation during operation and obtaining a precise length of sucker rods between the pump and the walking beam) create significant problems in matching up the sucker rod string both at the time of installation and during operation. This is a problem at the time of installation, and it may very well become a problem long after installation has occurred. For instance, sucker rods do elongate with use, and the sucker rod string may therefore have to be shortened.
There are hydraulic operated walking beam mechanisms. They operate primarily as a replacement for the power plant which has been heretofore a walking beam power plant.
The present invention is directed to an improvement in power plants for a downhole pump. It is particularly advantageous in that it is able to be connected directly to the casing at the well head and thereby enable the device to attach directly to the well head at the Christmas tree location, permitting direct connection of the sucker rod string to a polished rod as disclosed herein, thereby permitting adjustments in the sucker rod elevation. Moreover, the length of stroke can be adjusted quite readily.
All of these adjustments are accomplished with a view of providing a hydraulically powered well pumping apparatus which overcomes the problems mentioned above. That is, it can be installed directly on the top of the casing at the well head and operates in such a manner that a polished rod stuffing box is not required. Not only does installation proceed in a more easy fashion, the device further enables the polished rod to be installed in the sucker rod string without going to special efforts to cut and trim sucker rods to obtain a precise length. Further, the length of stroke is adjustable. Because of these adjustments, the problems arising in the installation are markedly reduced. Should there be permanent elongation of the sucker rod string, that also can be accommodated at the time of workover, thereby enabling the relative location of the pump at the end of the string to be modified or the length of stroke to be modified, these two adjustments being accomplished quite easily in the field without pulling the entire sucker rod string. This is advantageous at workover, months or years after the sucker rod string has been installed in the well.
With the foregoing in view, one feature of this apparatus is to provide a hydraulic powered well pumping system for operation of a downhole pump connected to the pumping apparatus by sucker rod string. Through the use of a pressure accumulator in the hydraulic system, energy peaks are reduced. Rather than require large energy peaks on the upstroke (lifting thousands of feet of sucker rods), the device operates at a more constant load by incorporation of a hydraulic accumulator in the high pressure side of the power system. The accumulator is filled and pressurized to a very high pressure. When the sucker rod string is lifted hydraulically, the accumulator dumps oil into the hydraulic lifting apparatus, thereby hoisting the entire sucker rod string. This is accomplished with a reduced peak load. It enables the reduction of the power plant capacity by perhaps 40% and maintains harmonic balance on the sucker rod string from great depth of 6,000 to 10,000 feet.
Other features and advantages of the present apparatus will be more readily apparent upon review of the detailed description which is set forth below. The apparatus is briefly summarized as a hydraulically powered piston in a cylinder surrounding a sleeve. The sleeve is adjustably fixed to a long polished rod. A hydraulic power system is also incorporated.